In recent years, fiber-optic cables have been increasingly used for communications, particularly in telephone systems. Reasons for this increased usage include the facts that optical fibers are lighter in weight and less expensive than electrical conductors, and are not subject to electrical interference. Typically, a communication system includes a light source such as a laser diode or LED, and a photodetector such as a silicon photodiode, connected through a single mode or multimode fiber optic cable. The source should ideally operate with a stable, single frequency output. Information is transmitted in digital form, as a series of light pulses that form a bit stream.
In order to increase the information-carrying capacity of a fiber-optic cable, frequency and time division multiplexing techniques have been widely explored in recent years. Examples of prior art frequency division multiplexing optical communication systems are described in U.S. Pat. Nos. 4,236,243 and 4,592,043. However, a number of problems have been encountered in implementing such systems. These problems include frequency variations of the semiconductor light sources, matching of the multiplexer and demultiplexer coupling frequencies, and the need for relatively large channel spacing to accommodate aging effects and manufacturing tolerances of semiconductor lasers.